The present invention relates to audio devices each provided with one or more Helmholtz resonators and also relates to methods for designing and making the audio devices.
Among the conventionally-known audio devices, including members corresponding to a neck and cavity of a Helmholtz resonator, such as sound absorbing panels are ones which are constructed to vary acoustic effects achieved thereby through adjustment of sizes of the members. Helmholtz resonance in the Helmholtz resonator is a phenomenon where, in response to sound waves of a resonant frequency fr of the Helmholtz resonator entering (or being introduced into) the neck, air within the neck violently vibrates together with air located in the neighborhood of the outer side of the neck so that energy of the introduced sound waves is reduced by being converted to heat on the inner peripheral surface of the neck.
Japanese Patent Application Laid-open Publication No. HEI-4-159898 (hereinafter referred to as “patent literature 1”) discloses a speaker system and more particularly a technique of varying a resonant frequency fr by adjusting a length of a member of a sound absorbing panel which corresponds to the neck of the Helmholtz resonator. The sound absorbing panel disclosed in patent literature 1 includes upper and bottom surface plates spaced opposed to each other via four side surface plates, and an accordion-type or bellows-type hose having one end opening in the upper surface plate and extending toward the bottom surface plate. In the disclosed sound absorbing panel, the bellows-type hose functions as the neck of the Helmholtz resonator, and a space interposed between the upper and bottom surfaces functions as the cavity of the Helmholtz resonator.
The Helmholtz resonator can be regarded as a mechanical-type single resonance system where air violently vibrating in response to sound waves of the resonant frequency fr being introduced into the neck is mass m and air within the cavity is a spring of a spring constant k, and relationship as indicated by Mathematical Expression (1) below is established among the resonant frequency fr, mass m and spring constant. k (see “Dictionary of Audio Terms New Edition”, Acoustical Society of Japan, Jul. 15, 2004, page 350)).fr=½π(k/m)1/2  (1)
Also, if the neck of the Helmholtz resonator has a cross-sectional area S, the cavity has a volume V and the neck has a length L, Mathematical Expression (1) above can be converted to Mathematical Expression (2) below, where c represents the speed of sound and ΔL represents an open end correction value to be added to the neck length L in order to fill a difference between the mass m of the air violently vibrating in response to sound waves of the resonant frequency fr being introduced into the neck and mass m′ of air within the neck (m′<m).fr=(c/2π){S/[(L+ΔL)V]}1/2  (2)
In the Helmholtz resonator, as shown in Mathematical Expression (2), the resonant frequency fr becomes higher as the neck length L is reduced, while the resonant frequency fr becomes lower as the neck length L is increased. Thus, with the technique disclosed in patent literature 1, the frequency of a sound to be absorbed becomes higher as the hose is reduced in length (L) and becomes lower as the hose is increased in length (L).
However, the technique disclosed in patent literature 1 would present the problem that designing and making the sound absorbing panels requires time and labor, because the sound absorbing panels are complicated in construction as compared to counterparts where the hose is fixed in length.